In recent times the use of optical fibers to send optical signals carrying the information from a transmission station to a receiving station has become well known in the telecommunication field.
Optical signals sent over an optical fiber undergo attenuation so that it may be necessary to amplify the signal level at the side of the transmission station by a booster optical amplifier.
An optical preamplifier is usually made available at the receiving station to bring the signal level to a range of values that is appropriate for a receiving apparatus. Known optical amplifiers are those based on the properties of a fluorescent dopant (erbium for example) that, if suitably excited by administration of luminous energy, gives a high emission in the wavelength band corresponding to the minimum light attenuation in silica-based optical fibers.
Optical fiber amplifiers, such as erbium doped optical fiber amplifiers are known, for example, from the patent application EP 0 677 902.
Many of the optical fiber links presently in use operate with a limited throughput, in comparison to the data transmission rates that can be reached by available transmitting and receiving equipments. For example, bit rates lower than 1 Gbit/s are in use for transmission of digital signal, while transmitting and receiving equipments working at 2.5 Gbit/s or at even longer rates are currently available. To allow an error free transmission on existing lines with an increased data throughput, the signal to noise ratio at the receiver station should be correspondingly increased. This can be achieved, for example, by increasing the level of the signal at the receiving station. For a given line, a 3 dB signal level increase allows to double the bit rate of the transmitted signal with the same bit error rate.
This can be done by providing line amplifiers at one or more locations at predetermined intervals along the fiber, to periodically raise the power of the transmitted optical signal.
Line amplifiers, however, are expensive and require maintenance. Further, addition of line amplifiers may not be feasible along existing fiber lines, such as e.g. for fiber lines lodged in submarine cables, or in other cables of difficult access.
The Applicant was faced with the problem of providing an increased data throughput in existing high span loss telecommunication lines, without adding line optical amplifiers.
In particular, the Applicant has observed that no known optical booster amplifiers can be used in a telecommunication system in which two signals at a bit rate of 2.5 Gbit/s are transmitted on a single span of optical fiber having an attenuation of 58 dB and an expected signal at the input of the preamplifier of -38 dBm.
The Applicant has perceived the possibility of solving the problem by providing a booster with a increased output power.
To obtain a booster with such performances, namely a very high output power, the Applicant considered to increase the pumping power applied to the optical active fiber.
Lasers, ad in particular laser diodes are convenient sources for pumping erbium single mode amplifiers, but the power available from a typical laser diode is limited.
For the above and other reasons it is difficult to provide high output power optical amplifiers.
Further a high output power optical amplifier should be relatively noise free, in particular for CATV (cable television) applications.
The patent U.S. Pat. No. 5,140,456 discloses a rare earth doped optical fiber amplifier pumped at a first wavelength selected to provide a low noise figure, and at a second wavelength selected to provide a high power efficiency. An erbium doped fiber amplifier is illustrated, in which the first wavelength is about 980 nm and the second wavelength is about 1480 nm. The travelling wave erbium fiber amplifier illustrated operates like a two stage amplifier, although there is an overlapping of the stages.
Other optical amplifiers are disclosed in the patent applications EP 497 246 and EP 508 880.
The patent U.S. Pat. No. 5,623,362 discloses a 980 nm/1480 nm band hybrid pumped fiber amplifier with a pump light isolator, installed between the output portion of the first optical fiber and the input portion of the second optical fiber of an amplifier. From the graphics reported in the patent a maximum output power of about 20 dBm has been obtained with 200 mW of total pump power.
The patent U.S. Pat. No. 5,287,216 discloses a doped fiber which is simultaneously pumped by multiple pump lasers generating optical waves of different wavelengths, (in the 960-1000 nm wavelength window). Of the two most common types of pump lasers, 1480 and 980 nm, the later is more efficient and also results in lower noise in the amplifier, making it the wavelength of choice from the perspective of performance. The use of multiple pump lasers decreases the power requirements of each laser, reduce the cost of the amplifier, and increases reliability without compromising the gain of the amplifier. The patent U.S. Pat. No. 5,185,826 discloses a cascaded hybrid pumping arrangement for doped fiber amplifiers.
The article of Y. Tamura, S. Shukii, Y. Kawai with the title "Semiconductor laser pump module with 240 mW output around 1.47 .mu.m band", IOOC 89, Kobe, Japan, discloses that a pump module having output of 240 mW was achieved in a single mode fiber by coupling the output of four 1.47 .mu.m band semiconductor lasers using wavelength division and polarizing multiplexing.
The Applicant has observed that polarization multiplexing technique is applicable only for combining lasers in the 1480 nm band, and not to other bands, such as, e.g. the 980 nm band, because no polarization maintaining fibers are available for this band.
The polarization multiplexer technique requires an emission of orthogonal polarized lights from two lasers and the use of polarization maintaining fibers. The two orthogonal polarized lights have to be multiplexed with a multiplexing device as a polarization beam combiner (PBC). The insertion loss of the PBC is relatively high, more than 1 dB, and the output power from the amplifier will suffer because of this high insertion loss.
None of the amplifiers described in the cited documents have the required performances.